Various types of adjustable-diameter pulley transmissions have been developed and used. In general, such transmissions take the form of two adjustable pulleys, each pulley having a sheave which iw axially fixed and another sheave which is axially movable relative to the first sheave. A flexible belt of metal or elastomeric material is used to intercouple the pulleys. The interior faces of the pulley sheaves are beveled or chamfered so that, as the axially displaceable sheave is moved, the distance between the sheaves and thus the effective pulley diameter is adjusted. The displaceable sheave includes a fluid-constraining chamber for receiving fluid to increase the effective pulley diameter, and when fluid is exhausted from the chamber, the pulley diameter is decreased. Generally the effective diameter of one pulley is adjusted in one direction as the effective diameter of the second pulley is varied in the opposite direction, thereby effecting a change in the drive ratio between an input shaft coupled to an input pulley and the output shaft coupled to the output pulley.
Various refinements were gradually evolved for the hydraulic control system which is used to pass fluid into the fluid-holding chamber of each adjustable pulley. One example of such a hydraulic control system is U.S. Pat. No. 3,115,049--Moan. The patentee utilizes two valves to control the secondary pulley adjustable sheave, to regulate the belt tension, and one valve to regulate fluid into and out of the primary sheave chamber, to regulate the transmission ratio. Subsequently U.S. Pat. No. 4,152,947--van Deursen et al was issued, teaching one valve to control the belt tension. In both these references the line pressure of the fluid applied to hold the belt tension by pressurizing the secondary chamber is relatively high. Thus an application entitled "Control System for Continuously Variable Transmission", Ser. No. 257,035, was filed Apr. 24, 1981, and assigned to the assignee of this invention; that application issued June 11, 1985 as U.S. Pat. No. 4,522,086. That application describes an improved control system which reduces the main line fluid pressure supplied to the secondary sheave chamber as a function of torque demand.
With these various control systems taught in the art, there still remains a need for a cost-effective, efficient control system which reduces the line pressure applied to the secondary sheave chamber to a low, but safe, operating pressure, and utilizes a lower control pressure for other portions of the hydraulic control system. It is to the provision of such an improved control system, with the lower pressure in a control line of the hydraulic control system, that the present invention is primarily directed.